LTE Physical Structure
3rd Generation Project Partnership (3GPP) Long Term Evolution (LTE) supports a type 1 radio frame structure, which is applicable to Frequency Division Duplex (FDD), and a type 2 radio frame structure, which is applicable to Time Division Duplex (TDD).
FIG. 1 shows the structure of a type 1 radio frame. The type 1 radio frame includes 10 subframes, each of which consists of two slots.
FIG. 2 shows the structure of a type 2 radio frame. The type 2 radio frame includes two half-frames, each of which is composed of five subframes, a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP), and an Uplink Pilot Time Slot (UpPTS), in which one subframe consists of two slots. The DwPTS is used for initial cell search, synchronization, and channel estimation at a User Equipment (UE). The UpPTS is used for channel estimation and uplink transmission synchronization of the UE at a Base Station (BS). The GP is used to remove interference occurring in uplink due to a multipath delay of a downlink signal between the uplink and a downlink. Meanwhile, one subframe consists of two slots regardless of the radio frame type.
FIG. 3 shows the structure of an LTE downlink slot. As shown in FIG. 3, a signal transmitted in each slot can be described by a resource grid including NRBDLNSCRB subcarriers and NsymbDL Orthogonal Frequency Division Multiplexing (OFDM) symbols. Here, NRBDL represents the number of Resource Blocks (RBs) in downlink, NSCRB represents the number of subcarriers constituting one RB, and NsymbDL represents the number of OFDM symbols in one downlink slot.
FIG. 4 shows the structure of an LTE uplink slot. As shown in FIG. 4, a signal transmitted in each slot can be described by a resource grid including NRBULNSCRB subcarriers and NsymbUL OFDM symbols. Here, NRBUL represents the number of RBs in uplink, NSCRB represents the number of subcarriers constituting one RB, and NsymbUL represents the number of OFDM symbols in one uplink slot.
A Resource Element (RE) is a resource unit defined as an index (a, b) in the uplink slot and the downlink slot and represents one subcarrier and one OFDM symbol. Here, ‘a’ is an index on a frequency axis and ‘b’ is an index on a time axis.
In legacy 3GPP LTE, a Reference Signal (RS) pattern of an antenna port 5 is defined as a downlink RS. The RS pattern of the antenna port 5 is named a UE-specific RS. The UE-specific RS may be referred to as a Dedicated Reference Signal (DRS) and a Demodulation Reference Signal (DM-RS) corresponds to the DRS.
An LTE-Advanced (LTE-A) system evolving from the LTE system can improve throughput using a maximum of 8 transmission antennas as compared with 4 transmission antennas in LTE.
To effectively reduce RS overhead, a DRS may be used which is defined in units of RBs of a frequency domain, wherein the DRS is allocated to a UE. The DRS may be precoded and distinguishable patterns corresponding to the number of ranks or layers may be used. ‘Distinguishable patterns’ mean that the patterns are orthogonal. The DRS need not be precoded.
An RS for supporting backward compatibility and forward compatibility with the LTE system may be defined and an RS of an antenna port 5 of the LTE system may be used as a DRS in terms of reusing the RS.
However, in order to support rank-8 using 8 transmission antennas in an LTE-A system, a maximum of 8 distinguishable DRS patterns should be supported.
It may be difficult to define a maximum of 8 distinguishable DRS patterns by using precoding in an RS pattern of one antenna port 5 and by using multiplexing, such as Code Division Multiplexing (CDM), Frequency Division Multiplexing (FDM), Time Division Multiplexing (TDM), or a combination thereof, in a series of RS sequences. Accordingly, a method for defining a maximum of 8 distinguishable DRS patterns in a system supporting 8 transmission antennas is required.